Safety Control Technology of Ultra-High Pressure Gas Wells with Annular Pressure: Case Study of Tarim Oilfield

Kuqa foreland basin in Tarim Oilfield is the largest ultra-high pressure gas fields in the world, with buried depth of 6000-10000 m and reservoir pressure of 115-14 MPa. Complex geological conditions, extreme working conditions and harsh corrosion environment bring great challenges to the integrity of high-pressure gas wells. This paper introduces the safety control technology for the ultra-high pressure gas wells, which ensures the safe production. For economic development ultra-high pressure gas field, it is essential to control the safety of high annular pressure. Field investigation of abnormal pressure indicates the previous design method and pressure control measures of two-stage barrier decreasing step by step in ultra-high pressure gas wells are difficult to meet the requirements of fine safety control. Therefore, the annular pressure safety control technology with the core of "two-stage well barrier equal strength design, annular pressure dynamic window control and pressure grading control" is established to realize the fine control of ultra-high pressure gas wells. Firstly,in the stage of ultra-high pressure gas well wellbore design, based on the equal strength design method, 140 MPa metal seal easily replaceable casing head and small coupling production casing are developed, which are integrated with the equal strength tubing string and Christmas tree to realize the "double insurance" of wellbore pressure. Even if the primary barrier fails, the secondary barrier can still prevent the leakage of high pressure gas.Secondly, based on international standards such as API RP 90-2 and ISO16530, the calculation method and control chart of annular allowable pressure dynamic window were established before ultra-high pressure gas wells were put into production. To improve the scientific foundation of annular pressure control, the damaged or failure mode of well barrier was taken into account to guide the annular pressure management. Finally, a management method was established for the production of ultra-high pressure gas wells, including risk assessment process and hierarchical management, and the hierarchical response measures are formulated to ensure the safety and stability of oil and gas wells, which can avoid the high risk and high investment brought by work over operations. Based on the safety control technology of ultra-high pressure annulus pressure in Tarim Oilfield, the proportion of oil casing annulus pressure in new wells is reduced from 40% to 0% within one year, and the overall annulus abnormal pressure well is reduced from 29.4% to 19.7%, which ensures the effective development and safe production of ultra-high pressure gas fields in Kuqa foreland basin.